Orchard planting systems for apple have been developed according to high standards in recent years. During the past 60–70 years, planting density has increased from 100 trees/ha to sometimes 10,000/trees ha in an attempt to increase orchard productivity and profitability (Robinson, 2003, 2011). The success of intensive systems depends on balancing the management of tree architecture and yield. Early yields are mainly under the influence of the number of trees per hectare and of tree training on light interception (Loreti et al., 1976; Robinson, 1996). For temperate crops such as apple, the use of pruning, detailed training, and rootstocks to control tree size has allowed a reduction in tree spacing and has led to increased flowering points per hectare, resulting in improvements in early productivity (Robinson, 2004).
Increased maximum yield and improvements in fruit quality have also been made possible by focusing on structural architecture, using rootstock and scion selection, and detailed training and pruning to increase efficiency of the conversion of solar radiation to fruit production in intensive systems (Costes et al., 2006; Smart and Robinson, 1991). Intensive orchard systems with developed cultivars and advanced management techniques (e.g., pruning, limb training, trellising) can maintain high light interception, balance flowering and vigor, provide efficient harvesting, reduce production costs, and improve the consistency of fruit quality (Palmer, 2007).
Macadamia integrifolia, Macadamia tetraphylla, and hybrids are subtropical evergreen trees native to coastal southern Queensland and northern New South Wales, Australia. Vegetative growth is vigorous and quickly forms a dense canopy. Trees are traditionally planted at low density and can grow as tall as 20 m at maturity (Nagao et al., 1992), which is several times the height of modern apple orchard systems. In mature orchards, the tall and dense canopy can lead to reduced spray efficiency, and shading can result in soil erosion and unproductive areas in the lower canopy (Huett, 2004). Nonetheless, macadamia can maintain its yield in high levels of shading of up to 94% light interception (McFadyen et al., 2004), possibly due to long-distance assimilate transport from irradiated parts of the canopy (Trueman and Turnbull, 1994). Yield reduction begins at canopy volumes greater than 43,500 m3·ha−1 (McFadyen et al., 2004); eventually, a proportion of older orchards consists of an unproductive inner canopy due to low light levels. Current management techniques mainly rely on biomass removal using mechanical hedging, which is intended to maintain machinery access rather than canopy efficiency (McFadyen et al., 2011). Hedging may negatively affect yield by removing fruiting age wood (Olesen et al., 2011; Wilkie et al., 2009), stimulating competitive vegetative growth (Wilkie et al., 2010), and lowering the photosynthetic potential of newly exposed shade leaves (Huett, 2004). Selective limb removal has been shown to increase and to have no effect on or to decrease yield compared with unpruned trees (McFadyen et al., 2013; Olesen et al., 2011). The reduction in yield is related to the amount of biomass removed, which is probably related, at least partly, to reduced light interception (Olesen et al., 2011).
Controlled vigor, early flowering, high partitioning of resources to productive processes, and high yield per hectare are key attributes of intensive temperate orchards that will benefit future macadamia orchards. However, crowding, shading, and the refinement of advanced training and pruning techniques remain obstacles to subtropical crops such as macadamia and avocado (McFadyen et al., 2004, 2013; Menzel and Le Lagadec, 2014). The discovery and incorporation of less vigorous scion cultivars and dwarfing rootstocks may be the most effective means of controlling vigor and increasing canopy efficiency for macadamia; however, there is currently limited research regarding such cultivars and little understanding of canopy architecture specific to macadamia (Huett, 2004; Nagao et al., 1992). The training of limb angles toward the horizontal can reduce vigor and increase flowering of apple and pear (Han et al., 2007; Lauri and Lespinasse, 2001; Sherif, 2013), and increased branching can improve early flowering of mango due to availability of terminal shoots (Oosthuyse and Jacobs, 1995), although similar relationships are unclear for macadamia. Therefore, it is necessary to improve our understanding of the architecture and interactions between vegetative and reproductive growth specific to macadamia.
This study documented the diversity and development of macadamia canopy architecture and reproduction across genotypes to associate multiscale traits with canopy size and yield and to assess which traits are most valuable for high-density orchards. This study has provided initial understanding of interactions between architectural traits to stimulate improvements in canopy management techniques and trait selections for breeding to develop efficient high-density macadamia orchards.
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Means of architectural and yield traits for each genotype (n=3) over 2 years (2016–17).
Path analysis of relative trait associations with canopy volume. Bold traits indicate direct associations (underlined) with the complex trait as standardized partial regression coefficients. Other traits are associated indirectly and derived from r × β.
Path analysis of relative trait associations with yield. Bold traits indicate direct associations (underlined) with the complex trait as standardized partial regression coefficients. Other traits are associated indirectly and derived from r × β.